The key is situated between the power-on key
and the power-off key. It is spring-loaded, with the
spring restoring the key to the secure position.
In a multiprocessing system, the TOD clock is
secure only when the TOD clock on this CPU and
the keys on all CPUs configured to this CPU are in
the secure position. The TOD clock in each CPU of
a multiprocessing configuration is enabled for setting
when th,e TOD clock key on that CPU, or on any CPU configured to it, is in the enable-set position.
Wait lndicator
The wait indicator is on when the CPU is in the wait
state. Operation Note
The wait indicator, manual indicator, and system
indicator may be used by the operator to determine
the status of the system. The following table shows
the possible conditions when power is on and the CPU is not in the load or check-stop states.
Remote Operator-Control Panel
In some models, a remote operator-control panel (ROCP) is provided. The remote operator-control M9nual Indicator
1 System I ndicator
2
Wait Indicator off off off off off on off on off
off on on
on off off
on off on
on on off
on on on Explanation: * Abnormal condition.
panel functions are effective concurrently with those
on the system console. The settings of the load-unit­
address controls on the remote panel are selected if
the load key on the remote panel is activated.
Customer-Engineer Section
The customer-engineer section of the system control
panel contains controls intended only for customer­
engineer use.
Operation Note
Improper use of the customer-engineer-section con­
trols may, among other things, result in false
machine-check indications or cause actual machine
malfunctions to be ignored. It may also alter other
aspects of system operation, including instruction
execution and channel operation, to the extent that
the operation does not comply with that specified in
this manual. While the abnormal setting of such con­
trols causes the test indicator to be turned on in this CPU, in a multiprocessing system the operation of
another CPU may be affected even though its test
indicator is not turned on.
CPU State State of I/O System
3 Operating, Wait Not working
Operating Undetermined
Operating, Wait Working Stopped Not working Stopped, Wait Not working
Stopped Working
Stopped, Wait Working
1 The manual indicator may be off in some models while display-and-enter
operations are being performed.
2 When the system indicator is turned on, it remains on for a minimum of appro):imately one second.
3 The operation of the console device is included here as an I/O operation. In a
multiprocessing system, the system indicator may be on because of activity in
anothe'r CPU. When this is the case, the state of I/O system in undetermined.
System Status Indications
248 System/370 Principles of Operation

Central Processing Unit Every CPU incorporates the commercial instruction
set, which includes the standard instruction set and
the decimal instructions (listed in Appendix C), and
the associated basic computing functions, including: • Byte-oriented operands • General registers • Control registers, with bit positions "for the
block-multiplexing control bit (if block multi­
plexing is provided), for the interrupt-key and
interval-timer masks, for channel masks associ­
ated with installed channels, for monitor
masks, for control of installed machine-check­
handling facilities, and for the IOEL control (if
an installed channel has the I/O-extended­
logout facility). • Storage protection • Interval timer • Time-of -day clock • Basic system console functions
Additionally, the following features are available:
Floating-Point Feature
Includes the floating-point instructions (listed in
Appendix C) and the floating-point registers.
Universal Instruction Set Includes the instructions of the commercial instruc­
tion set and the floating-point feature.
Extended-Precision Floating-Point Feature
Includes the extended-precision floating-point in­
structions (listed in Appendix C).
External-Signal Feature
Includes the extension to external interruptions for
external signals, the control-register position for the
external-signal mask, and the means to accept exter- , nal signals.
Direct-Control Feature
Includes the external-signal feature and the instruc­
tions READ DIRECT and WRITE DIRECT.
CPU-Timer and Clock-Comparator Feature
Includes the clock comparator, the CPU timer, the
associated extensions to external interruption,
control-register positions for the clock-comparator
and CPU-timer masks, and these instructions: SET Appendix A. System/370 Features CLOCK COMPARATOR, STORE CLOCK COM­ PARATOR, SET CPU TIMER, and STORE CPU TIMER.
Translation Feature
Includes the following facilities: • Dynamic Address Translation (DAT). The
DAT facility includes the translation mecha­
nism, with the associated control-register posi­
tions and program-interruption codes, and ref­
erence and change recording. • Program-Event Recording (PER). The PER
facility includes the associated control-register
positions and extensions to the program­
interruption code. • Extended-Control (EC) Mode. • SSM Suppression. This facility includes the
control-register position for the SSM­ suppression-control bit and the program­
interruption code for special operation. • Store Status and Program Reset.
As part of these facilities, the following instruc­
tions are provided: LOAD REAL ADDRESS, PURGE TLB, RESET REFERENCE BIT, STORE THEN AND SYSTEM MASK, and STORE THEN OR SYSTEM MASK. Multiprocessing Feature
Includes the following facilities, which permit the
formation of a two-CPU multiprocessing system: • Shared Main Storage. • Prefixing. • CPU Signaling and Response. • TOD Clock Synchronization.
These facilities include four extensions to external
interruption (external call, emergency signal, TOD clock sync check, and malfunction alert), control­
register positions for the TOD-clock-sync control bit
and for the masks for the four external-interruption
conditions, and the instructions SET PREFIX, SIG­ NAL PROCESSOR, STORE CPU ADDRESS, and STORE PREFIX.
Conditional-Swapping Feature
Includes the instructions COMPARE AND SW AP
and COMPARE DOUBLE AND SWAP. PSW-Key-Handling Feature
Includes the instructions SET PSW KEY FROM ADDRESS and INSERT PSW KEY.
Appendix A. System/370 Features 249